Page 8 - Open access sep 2020

TECHNICAL PAPER

The different examples shown in Table 1 provide a glimpse of which can be removed later. In this study, the adjustment
mixes being studied and probable areas of application (like of printing parameters is made to print main and support
construction on the Moon and Mars). structures for overhang construction using single type building
material. A parameter called solidity ratio (best filament has
3. PROPERTIES AND APPLICATIONS OF 3D solidity ratio of 1) is introduced which is defined as the ratio
PRINTABLE CONCRETE between the actual and expected size of filament, which is
affected by material flow rate and speed of nozzle. A gap
3.1 Extrusion of 3D printable concrete distance characterization test is carried out to determine the
maximum distance between the supports to avoid slumping of
The mixes that are designed for 3D printing need to be filament. This paper also suggested a future scope to study the
extruded through a nozzle to attain the required shape. effect of material thixotropy on dimensional parameters of the
Extrusion, which depends on rheological properties of the filament and for easy removal of support structures.
material and geometrical configurations of the extruder, is
classified by Nerella et al. (2018) as full-width printing (extrudate Extrusion tests are important for the selection of materials like
is equal to the breadth of target layer) and filament printing geopolymer, foam concrete, etc. Markin et al. (2019) conducted
(breadth of the extrudate is much smaller than that of target extrusion and deposition tests using a stand-alone progressive
layer) based on layering technique. The improper extrusion of cavity pump and 3D concrete printing test device on foam
concrete can lead to over-extrusion or under-extrusion. Over concrete as described earlier. The extruded and deposited
extrusion will cause an increase in width from the targeted mixes during the first 20 minutes are reported to be free of any
value, and under-extrusion will lead to reduced layer width, visible deformation. The extrudable wood concrete developed
discontinuity, or even zero deposition at times. Since extrusion in TU Munich was extruded through a special kind of extruder,
is so important for further performance, real-time quality based on the concept of a household meat grinder. This
monitoring is proposed by Kazemian et al. (2019) by a vision- extruder is a modular system composed of an acrylic glass pipe
based technique. 2D images are analyzed in this system, making and conveyor screws with the motor. For industrial applications,
it less complex, and the reliability of the system was assessed a combined mixer-extruder unit is to be used (Henke et al.,
by exposing it to material variations. This system is to develop 2016).
a feedback-controlled closed-loop extrusion system which can The extrudability of geopolymer mortar was studied by Panda
be a precursor for automated quality monitoring of 3D concrete et al. (2018). The high static yield stress can negatively affect the
printing. extrudability, and high pressure is needed to initiate the flow,
but not always as cautioned in the study. Clogging is seen in
De Schutter et al. (2018) concluded that rheological and
stiffening properties are important aspects for 3D printing and this study and is attributed to the higher yield stress developed
active rheological control, and active stiffening control can due to friction generated between added fine aggregates
provide the solution for extrusion. Rheological control is much due to interlocking, and the need for further rheological study
needed for the extrudability of the material. The rheological in geopolymer concrete is suggested. Another important
properties were related to mix-design in a study by Nair et al. parameter discussed in the paper is ‘thixotropy open time’
(2019), and the extrusion geometry was related to the fresh (TOT), defined as the time interval beyond which extrudability
property is lost. The fly ash and ground granulated blast furnace
paste microstructure properties. The ratio of the solid volume slag (GGBS) quantity are varied in geopolymer to vary the TOT
fraction to the square of the median particle size was related
to yield stress and wall shear stress of 3D printable pastes. for selection of an extrudable mix with the property of shape
retention. Further modelling is needed to understand the
The compositional aspects such as particle size distribution, fundamentals of extrusion and its effect on different structures.
superplasticizer presence, etc. were also considered by Nair et
al. (2019). Modelling of extrusion-based printing of OPC pastes using
Discrete Element Modeling (DEM) considering Burger’s model
As workability is an important characteristic for conventional for the representation of particle interaction in normal and
concrete flow, in 3D concrete printing, extrudability and shear directions and creation of virtual extrusion experiments
buildability are related to workability and open time of the on paste for analysis shown in Yang et al. (2019). The rheological
concrete mix. Test methods for performance evaluation of a model is developed by extracting parameters from the mini-
range of mixes based on fresh properties were presented by slump test.
Le et al. (2012a). Tay et al. (2019) showed the importance of
rheological properties and operational speed of nozzle for the An algorithm to derive rheological properties was developed
construction of overhang structures using printed supports by researchers from the University of Seoul (Jeong et al., 2019)

12 THE INDIAN CONCRETE JOURNAL | SEPTEMBER 2020